You, like me, may have tried the method in the video posted by isaacballs. You, like me, probably grew frustrated wondering how that sixth guard triggered the explosion. I ran around like a maniac for at least 20 minutes trying to get said explosion. No luck.
Here's what I did that finally worked. I was on the last mission, 09 - The Light at the End. I used the platform in front of the wall of light to the right (the one without the turret) for my unconscious body stack.
In this area there are plenty of guards and several whale oil containers. There are at least two containers outside (by the arc pylon) and several inside (via blink and the drain in the whale - if you are going for "lights out", you will need to enter this area anyways).
Now get crafty and stack your five bodies. Heed the other advice to do this, I choked most my guys, but I stopped after 5. I made my stack in front of the stairs that lead down tot he water. I chose to leave the one guard, down by the water (way to the right of the platform/wall). This is the guy I need for the explosion.
Save now. You can use this stack for this achievement and the Tempest achievement.
For this achievement I held the whale oil container, got the guard to chase me, and as he came up the stairs I stood on top of my body pile with whale oil container in hand. The guard shot me, whale oil blew up, and the achievement unlocked. I managed to survive without a problem.
For Tempest, I put the whale oil container on top of the stack and when the guard came up the stairs and was close to the stack I shot the container. Boom, six down in under one second.
Two notes: This is a good mission for this as it seems all guards carry guns. Also, I may have had an extra whale oil container or two on the stack, in addition to the one I was carrying. That's why it's a good idea to save before trying to get the guard to shoot at you.